We previously demonstrated strain differences in susceptibility to radiation-induced mammary cancer and radiation-induced cytogenetic instability between sensitive BALB/cByJ, and resistant C57BL/6ByJ mice and provided evidence that such susceptibility differences were heritable traits. Sequence analysis of cDNA from these strains indicates two polymorphisms unique to the BALB/c mouse. Studies in the two strains, F1 hybrids, and in backcross animals have demonstrated a significant association between susceptibility to radiation-induced cytogenetic instability and Prkdc, the gene encoding the catalytic subunit of DNA dependent protein kinase (DNA PKcs). Mice homozygous for the BALB/c Prkdc allele (Prkdc BALB/c) are also deficient in the post-irradiation repair of double strand breaks, showed a reduced kinase activity, and western analyses showed a reduced intensity of signal for DNA-PKcs. Since the repair deficiency, reduced western signal intensity, reduced kinase activity, and the presence of these two polymorphisms are all significantly associated with increased susceptibility to radiation-induced cytogenetic instability, we now propose to examine the genetic linkage between this locus and susceptibility to radiation-induced mammary cancer, and to more directly elucidate the role of Prkdc BALB/c in radiation-induced instability and mammary cancer. We have now also demonstrated that irradiation of mammary cells from BALB/c as well as SCID, which have a truncating mutation of Prkdc, results in fusions between telomeres and radiation-induced double strand breaks (DSB) presumably as a result of a defect in the function of DNA PKcs as an important element in telomere maintenance. Specifically, the aims of this project are to: 1) directly determine the functional consequence of polymorphorisms in PrkdcBALB; 2) determine the role of Prkdc BALB in susceptibility to radiation-induced mammary cancer; 3) determine the contribution of telomere dysfunction in radiation-induced cytogenetic instability and mammary cancer.